1. Field of the Invention
The present invention relates to molecular beam epitaxial formation of semiconductor layers, and, more particularly, to a method for generating source beams having a high fractional content of monomeric (atomic) species.
2. Description of Related Art
Molecular beam epitaxy (MBE) is an ultrahigh vacuum technique for depositing single crystal semiconducting, insulating, and metallic materials. The advantages of MBE over other growth techniques include the ability to produce high purity materials with precise control of composition, layer thickness, and dopant (impurity) concentrations. In MBE, growth is achieved by directing "molecular" beams of the desired constituents onto a heated substrate in an ultrahigh vacuum chamber. These source beams traditionally are formed by heating a small reservoir that contains the relevant material in solid form so that sublimation or evaporation occurs. The design of the crucible that holds the source material, in con]unction with apertures external to the crucible, result in a directed beam of neutral molecules that impinge on the substrate at a rate that is controlled by the crucible temperature.
Thermal "effusion" sources of the type just described produce, in general, a mixture of both atoms and molecules that contain the atomic species of interest. Sources for elements from columns II, III, and IV of the Periodic Table tend to produce primarily atoms, whereas sources for elements from columns V and VI contain primarily tetramer (4-atom) and dimer (2-atom) clusters. Since the growth of a crystalline layer at the substrate surface ultimately requires individual atoms, dimers and tetramers must somehow be separated into atoms in order for epitaxial growth to occur.
Current practice relies on thermal energy from the substrate to "crack" the clusters on the surface and thereby separate them into individual atoms. However, this surface cracking process is very temperature dependent, and can be quite inefficient, requiring large excess incident fluxes of molecules to achieve the required incorporation of the species of interest. In some cases, it is not possible to find a substrate temperature at which all the necessary atomic species are efficiently produced at the surface, and thus it is essentially impossible to grow certain materials using conventional thermal effusion sources. An example of this type of limitation occurs in the growth of III-V ternary alloys containing two Group V constituents, such as GaAs.sub.x Sb.sub.1-x. MBE growth of these materials using conventional thermal sources for As and Sb has been largely unsuccessful due to an inability to control the relative proportions of the Group V elements in the crystal. This problem is directly linked to the molecular nature of the sources and their consequent inefficient incorporation into the growing crystal. The availability of atomic sources for As and Sb would eliminate the problem and permit the growth of high quality crystals of such materials.
The problem of non-atomic source beams has been recognized in the past, and some improvement has been achieved by adding a high-temperature "cracking" tube at the output of a conventional thermal effusion cell in order to achieve some conversion of tetramers to dimers. This has improved the control of MBE growth of crystals containing Group V species, but does not approach the level that could be achieved using pure monomeric (atomic) beams. In principle, if the cracking tube could be operated at a sufficiently high temperature, then the complete reduction of clusters to atoms would occur. However, in practice, the high temperatures required to produce a substantial atomic fraction from such a source are incompatible with the ultrahigh vacuum growth environment required to produce high purity crystals. For this reason, thermal cracker cells are usually employed only to reduce tetramers to dimers, and are not effective for producing beams that are substantially monomeric.
MBE is also practiced in another form in which gaseous sources are used instead of or in addition to effusion cells to form the source beams. Sources for gas-phase MBE, including metal-organic MBE (MOMBE), are typically metal organic or metal hydride molecules. Thermal dissociation of the molecules on the growth surface is usually relied upon to liberate the metal atoms. However, the thermal dissociation reaction is often very inefficient at the desired substrate (growth) temperature. Moreover, carbon-containing organic fragments produced by the dissociation can result in carbon contamination of the growing crystal. In an effort to circumvent these limitations, a thermal cracker tube can be used with a gas source to crack the source molecules before they reach the substrate. However, in the case of Group V and VI metal-organics, it has been found that the metal atoms recombine within the cracker tube to form dimers and tetramers. Thus, this type of source suffers from the same problem as the thermal effusion cells used in conventional MBE, namely, that it does not produce an atomic beam.
Accordingly, there remains a need for a means to provide a significantly high fraction of monomeric (atomic) species, and to minimize dimeric and tetrameric species in the source beams used for MBE.